It has already been suggested to drive a central date indicator by a crown wheel by means of which one may add the date indication to a standard watch without the necessity of effecting important modifications to an existing movement. Swiss patent document No. CH-A-332 898 shows such a construction.
The arrangement as described calls for a mechanism arranged entirely above the dial, this having the disadvantage of increasing considerably the thickness of the watch. This same arrangement also calls for an elastic blade united with the pipe of the hours wheel which every twelve hours is lifted by a dog fixed to the dial in order to bring the free end of said blade into mesh with the teeth of the crown wheel, thus causing said crown wheel to advance by one step along with the date indicator which is fixed thereto. Such mechanism requires a substantial turning moment at the instant of driving of the indicator, such being ill advised for a watch driven by a stepping motor itself energized by a battery. Furthermore one may reasonably fear jamming of the crown wheel on the hour wheel pipe at the moment that the indicator is to be advanced.
In modern calendar watches, the date is indicated most often by means of a ring on which are inscribed figures 1 to 31 appearing in an aperture provided in the dial. Nowadays there is a strong demand, claiming once again the date display by means of a central hand rotating over an hours circle showing thirty-one divisions as was the case in ancient calendar watches. One may thus be tempted to adapt a mechanism foreseen for driving a date ring in a manner such that it may drive a central hand and this, as far as possible, in adding to the mechanism a minimum of modification.
Such an attempt is described in Swiss patent document No. CH-A-661 170. In this document the date indicating hand faces a fixed date circle appearing at the periphery of the dial and is borne by a pipe of a plate coaxial to the center of the movement on which is set a coupling piece exhibiting three elastic arms snap engaged with the heads of studs borne by the date ring.
The construction which has just been briefly described maintains use of the date ring (furthermore driven in a standard manner), this necessitating, in addition to manufacture of the connection piece with three arms, a special preparation of the ring as well as the manufacture of relatively complicated studs in order to couple such ring to the connection piece. Furthermore, the mere presence of the mentioned studs leads to a construction which assumes a certain thickness in addition to the thickness necessary for the development of the connection piece which is superposed.